Add SourceViewer#computeStyleRanges() to highlight external documents

Introduces computeStyleRanges(IDocument, IRegion) on SourceViewer,
which applies the viewer's own presentation reconciler and partitioner
to an external document, returning the resulting SWT StyleRanges for
the given region without affecting the viewer's current document.

Author: tobias-melcher

bundles/org.eclipse.jface.text/META-INF/MANIFEST.MF

@@ -2,7 +2,7 @@ Manifest-Version: 1.0
 Bundle-ManifestVersion: 2
 Bundle-Name: %pluginName
 Bundle-SymbolicName: org.eclipse.jface.text
-Bundle-Version: 3.30.100.qualifier
+Bundle-Version: 3.31.0.qualifier
 Bundle-Vendor: %providerName
 Bundle-Localization: plugin
 Export-Package: 

bundles/org.eclipse.jface.text/src/org/eclipse/jface/text/source/SourceViewer.java

@@ -24,6 +24,7 @@
 import java.util.Stack;
 
 import org.eclipse.swt.SWT;
+import org.eclipse.swt.custom.StyleRange;
 import org.eclipse.swt.custom.StyledText;
 import org.eclipse.swt.graphics.Point;
 import org.eclipse.swt.graphics.Rectangle;
@@ -46,7 +47,9 @@
 import org.eclipse.jface.text.IAutoEditStrategy;
 import org.eclipse.jface.text.IBlockTextSelection;
 import org.eclipse.jface.text.IDocument;
+import org.eclipse.jface.text.IDocumentExtension3;
 import org.eclipse.jface.text.IDocumentExtension4;
+import org.eclipse.jface.text.IDocumentPartitioner;
 import org.eclipse.jface.text.IPositionUpdater;
 import org.eclipse.jface.text.IRegion;
 import org.eclipse.jface.text.IRewriteTarget;
@@ -56,8 +59,11 @@
 import org.eclipse.jface.text.ITextSelection;
 import org.eclipse.jface.text.ITextViewerExtension2;
 import org.eclipse.jface.text.ITextViewerLifecycle;
+import org.eclipse.jface.text.ITypedRegion;
 import org.eclipse.jface.text.Position;
 import org.eclipse.jface.text.Region;
+import org.eclipse.jface.text.TextPresentation;
+import org.eclipse.jface.text.TextUtilities;
 import org.eclipse.jface.text.TextViewer;
 import org.eclipse.jface.text.codemining.ICodeMiningProvider;
 import org.eclipse.jface.text.contentassist.IContentAssistant;
@@ -71,6 +77,8 @@
 import org.eclipse.jface.text.hyperlink.IHyperlinkDetector;
 import org.eclipse.jface.text.information.IInformationPresenter;
 import org.eclipse.jface.text.presentation.IPresentationReconciler;
+import org.eclipse.jface.text.presentation.IPresentationReconcilerExtension;
+import org.eclipse.jface.text.presentation.IPresentationRepairer;
 import org.eclipse.jface.text.projection.ChildDocument;
 import org.eclipse.jface.text.quickassist.IQuickAssistAssistant;
 import org.eclipse.jface.text.quickassist.IQuickAssistInvocationContext;
@@ -1393,4 +1401,77 @@ private void uninstallTextViewer() {
 		lifecycles.clear();
 	}
 
+	/**
+	 * Computes syntax-highlighting style ranges for a region of an external document using this
+	 * viewer's configured presentation reconciler and partitioner.
+	 * <p>
+	 * This is useful when you want to syntax-color content that is <em>not</em> the document
+	 * currently displayed in the viewer — for example, to preview or render a snippet with the same
+	 * language rules that are active in this viewer.
+	 * </p>
+	 * <p>
+	 * The viewer's partitioner is temporarily connected to the given document so that its
+	 * presentation repairers can compute the correct highlighting. The viewer's own document is not
+	 * affected.
+	 * </p>
+	 *
+	 * @param document the external document whose content should be highlighted; must not be
+	 *            {@code null} and must use the same language/content type as this viewer
+	 * @param damage the region within {@code document} for which style ranges are computed; must
+	 *            not be {@code null}
+	 * @return the list of {@link org.eclipse.swt.custom.StyleRange}s covering the given region, as
+	 *         produced by this viewer's presentation reconciler; never {@code null}, may be empty
+	 *         if no repairer is registered for the content type
+	 * @throws BadLocationException if {@code damage} is outside the bounds of {@code document}
+	 * @since 3.31
+	 */
+	public List<StyleRange> computeStyleRanges(IDocument document, IRegion damage) throws BadLocationException {
+		String partition= IDocumentExtension3.DEFAULT_PARTITIONING;
+		IPresentationReconciler reconciler= fPresentationReconciler;
+		if (reconciler instanceof IPresentationReconcilerExtension ext) {
+			String extPartition= ext.getDocumentPartitioning();
+			if (extPartition != null && !extPartition.isEmpty()) {
+				partition= extPartition;
+			}
+		}
+		IDocument originalDocument= getDocument();
+		IDocumentPartitioner partitioner= originalDocument.getDocumentPartitioner();
+		document.setDocumentPartitioner(partitioner);
+		IDocumentPartitioner originalDocumentPartitioner= null;
+		if (document instanceof IDocumentExtension3 ext
+				&& originalDocument instanceof IDocumentExtension3 originalExt) {
+			originalDocumentPartitioner= originalExt.getDocumentPartitioner(partition);
+			if (originalDocumentPartitioner != null) {
+				// set temporarily another document in partitioner so that presentation can be
+				// created for given source
+				originalDocumentPartitioner.disconnect();
+				try {
+					originalDocumentPartitioner.connect(document);
+				} finally {
+					ext.setDocumentPartitioner(partition, originalDocumentPartitioner);
+				}
+			}
+		}
+		TextPresentation presentation= new TextPresentation(damage, 1000);
+		ITypedRegion[] partitioning= TextUtilities.computePartitioning(document, partition, damage.getOffset(),
+				damage.getLength(), false);
+		for (ITypedRegion r : partitioning) {
+			IPresentationRepairer repairer= reconciler.getRepairer(r.getType());
+			if (repairer != null) {
+				repairer.setDocument(document);
+				repairer.createPresentation(presentation, r);
+				repairer.setDocument(originalDocument);
+			}
+		}
+		if (originalDocumentPartitioner != null) {
+			originalDocumentPartitioner.connect(originalDocument);
+		}
+		List<StyleRange> result= new ArrayList<>();
+		var it= presentation.getAllStyleRangeIterator();
+		while (it.hasNext()) {
+			StyleRange next= it.next();
+			result.add(next);
+		}
+		return result;
+	}
 }